The present invention relates generally to signal transmission, and more specifically to an apparatus and method for broadcasting digital signals representing audio, video, data, etc. via satellite to multiple cable television systems and other terrestrial distribution systems which have multiple modulation schemes.
Terrestrial distribution systems include cable television systems ("cable system") which distribute cable television programs ("cable programs") to cable subscribers over coaxial cable or fiber-optic cable. They offer multiple television channels in areas where broadcast is difficult to receive, or supplement existing broadcast service by providing additional services such as pay-per-view. A standard analog cable TV system utilizes frequency division multiplex with multiple television channels, each with 6-Mhz bandwidth (hereinafter referred to as 6-Mhz cable slots).
Terrestrial distribution systems including cable systems must soon convert to digital distribution for providing digital video signals to cable subscribers, because of many advantages associated with digital systems, such as error correction, data compression, flexibility, programmability, and increased quality and quantity of service. These advantages are evidenced by the increased popularity of digital services such as direct digital broadcast service (DBS), multipoint multichannel distribution service (MMDS), etc.
Digital cable systems use a modulation scheme called quadrature amplitude modulation (QAM) to send signals over cable. QAM is a modulation method using both phase and amplitude modulation of a carrier in order to represent a number of information signals. For example, 64-QAM refers to the use of 64 different combinations of phase and amplitude to represent 64 different states of a symbol, or 6 bits (2.sup.6 =64) of data per symbol.
Current digital cable systems support 64 QAM over the existing 6-Mhz cable slot. This enables approximately 27 megabits per second (Mbps) of data per channel to be transmitted down the cable to the subscriber's home per 6-Mhz cable slot. For this reason, most suppliers of prepackaged digital satellite programming have configured their satellite transponders, each typically having a bandwidth of 36 or 27 Mhz, to deliver 27 Mbps per transponder.
Cable operators originate some cable programs, but they are mainly distributors of prepackaged cable programs received from cable program suppliers. Such cable programs are transmitted from the cable program supplier to the cable operator through a point-to-multipoint link such as a satellite link.
Communication satellites have been used for many years to transmit and distribute such cable programs to cable systems over larger geographical areas. The transmission paths from ground to satellite and back to ground are called an uplink and a downlink respectively. The carrier frequencies for uplink and downlink are usually different to avoid interference between the two. For example, in the C band used for satellite communication, the uplink frequency is in the 6 Ghz range and the down-link frequency is in the 4 Ghz range.
FIG. 1 shows a prior art digital cable system using satellite transmission to deliver the programming to the cable headend. In order to correct errors during transmission to and from the satellite, a transmitting earth station has a satellite transmitter 1 which encodes digital information signal 2 containing cable programs using a forward error correction (FEC) scheme. FEC refers to an error correction scheme using a redundant code based upon which errors can be detected and corrected without requesting a retransmission from the transmitter. FEC contrasts with automatic repeat request (ARQ), which enables error detection but not correction, where the receiver alerts the transmitter when errors occur so that the data can be retransmitted. FEC, in contrast, corrects errors at the receiving end without having to retransmit the data. Several FEC codes are well-known in the art, including block codes such as Hamming codes or Reed Solomon codes, and non-block codes such as convolutional codes. FEC is particularly suitable for transmission via satellite because it is generally impractical or impossible to request a retransmission of corrupted data.
A satellite-FEC encoder 3 converts the information signal to an FEC-encoded signal 4. A typical satellite-FEC encoder incorporates two levels ("shells") of error correction: a Reed Solomon outer shell for correcting byte errors and a convolution encoder inner shell for bit error correction.
The satellite transmitter in the earth station also includes a satellite modulator 5 to modulate the FEC-encoded signal 4, containing the cable programs, into a satellite signal 6. The signal 6 is sent to satellite 9 via antenna 7, producing uplink signal 8 in the form of a narrow beam at the uplink frequency.
Typically, a digital satellite signal is quaternary phase-shift key (QPSK) modulated. Unlike cable systems using QAM, which varies both carrier phase and amplitude, the satellite system typically uses phase-shift keying (PSK), varying only the phase of the carrier, because satellite systems are highly subject to amplitude fluctuations due to noise in the atmospheric channel. QPSK is a particular PSK modulation scheme which assigns two bits to a symbol having four (2.sup.2) possible phase states corresponding to 0, 90, 180, and 270 degrees. Thus, QPSK carries two bits per symbol.
The satellite 9 has a transponder 10 which receives the uplink signal 8 from the transmitting earth station, and amplifies and translates it into a downlink frequency for retransmission to a receiving earth station via downlink signal 11. A typical satellite currently used for this purpose may have 24 transponders, each supporting a bandwidth of approximately 27 or 36 Mhz.
A receiving earth station receives the downlink signal 11 through a receiving antenna 12 to generate a signal 13 which is QPSK modulated. The receiving earth station has a transcoder 14 which converts, or remodulates, the QPSK-modulated satellite signal 13 to a QAM-modulated signal for cable transmission. The transcoder is also known in the art as an integrated receiver transcoder (IRT). The prior art transcoder 14 includes a satellite demodulator 15 for demodulating the received satellite signal. A satellite-FEC decoder 17 decodes the resulting demodulated signal 16 by removing the redundant FEC codes (added for satellite transmission) to produce information signal 18. Similar to the satellite-FEC encoder 3, a typical satellite-FEC decoder incorporates two levels or shells of error correction: a Reed Solomon decoder outer shell for correcting byte errors and a convolution decoder inner shell for bit errors. A Viterbi decoder may also be used, which is a special kind of convolutional decoder known in the art.
In digital cable systems, the information signal 18 containing cable programs is then encoded using a forward error correction (FEC) scheme before it is modulated for transmission over cable.
For this purpose, a cable-FEC encoder 19 encodes the received information signal 18 to an encoded signal 20. Prior to the FEC-encoding, the signal may be encrypted to prevent unauthorized access (not shown).
A 64-QAM cable modulator 21 then modulates the encoded signal 20 to a cable signal 22 for 64-QAM transmission. For more details on the IRT, see General Instrument's "IRT 1000 Integrated Receiver Transcoder, Installation and Operation Manual," 1996.
As mentioned before, current digital cable systems generally support 64 QAM over the existing 6-Mhz cable channel. However, recent advances in technology enable the use of 256 QAM for cable transmission and distribution, allowing an increased data rate of approximately 38.8 Mbps (contrasted with the current 27 Mbps) per channel through the existing 6-Mhz cable channel to the subscriber's home.
Satellite transponders will thus need to be configured to deliver 38.8 Mbps per channel for 256-QAM cable transmission instead of 27 Mbps per channel for the previously used 64-QAM transmission in order to maximize the use of cable bandwidth.
This means, importantly, that cable operators and cable program suppliers must upgrade their equipment from 64 QAM to 256 QAM, at significant expense. Inevitably, some cable operators will find it impractical to upgrade their equipment at the precise instant when cable program suppliers start broadcasting to 256-QAM cable equipment. Indeed, it is wholly impractical to think that in this industry all concerned can agree upon a single instant when all programming will shift from that feeding 64-QAM to that required for 256-QAM distribution. An alternative is to duplicate the satellite channel by doubling the transponder capacity and related equipment, one for 64-QAM and the other for 256-QAM. However, this requires adding another expensive FEC encoder for each channel in the transmitter end. Thus it would be highly desirable to provide transmitting equipment capable of supporting both 64 QAM and 256 QAM simultaneously, thus permitting cable operators who have not upgraded to receive for 64-QAM and those that have upgraded to receive for 256-QAM.
When the cable program suppliers convert their transmission to the 256-QAM format, those cable operators still without the upgraded equipment will need to convert the received signals in the 64-QAM format. Since the 256-QAM format has a higher bit rate, it does not match with the 64 format having a lower bit-rate. The mismatch creates an excess data stream to be processed. Prior art transcoders are incapable of accommodating such a need. They do not have facility to siphon off the excess data stream, and thus generate a single data stream supporting only a single QAM format.
Therefore, there exists a need for apparatus capable of supporting the use of two QAM formats simultaneously.